Recently, glass flakes have received an increased attention as a substrate for effect pigments, particularly pearlescent pigments. Glass flakes or glass platelets have a very smooth surface and can be produced with rather uniform thickness, compared to mica substrates traditionally used as a substrate in pearlescent pigments (WO 2005/063637 A1). Pearlescent pigments based on glass flake substrates exhibit improved optical properties such as stronger chroma, higher colour purity and higher gloss when compared to traditional mica based pigments.
U.S. Pat. No. 3,331,699 discloses a pigment composition consisting essentially of glass flakes on which a very thin nucleating surface of a metal oxide compound is deposited at first. Subsequently, high refractive metal oxides are precipitated on said thin nucleating surface. The nucleating layer can be comprised of tin oxide or aluminium oxide hydroxide and is necessary to improve the adhesion of the subsequently applied metal oxide layers. As shown in WO 02/090448, the effect pigments obtained according to the teaching of U.S. Pat. No. 3,331,699 are poor in quality with respect to today's requirements and expectations.
U.S. Pat. No. 2,863,783 discloses the utilization of platelets or lamellae of glass in nacreous compositions. A glass lamellae is described with a content of TiO2 of 50.4% and a refractive index of 1.80. Such a composition has a refractive index to high to be used as a substrate material of pearlescent pigments. Furthermore, it has a very high softening point and is very expensive due to the high content of TiO2.
JP 7-246366 discloses pearlescent pigments on the basis of C glass flakes coated alternately with layers of titanium dioxide and silica. Pursuant to the teaching of JP 7-246366 tin oxide is also used to improve the adhesion of the layer of TiO2.
EP 0 912 640 B1 also discloses a pearlescent pigment comprising flakes of C glass having a first coating comprising iron oxide or rutile titanium dioxide thereon. Pursuant to the teaching of EP 0 912 640 B1 it is not possible to obtain pearlescent pigments having a high quality on the basis of flakes of A glass or E glass coated with metal oxide. The skilled person reads from EP 0 912 640 B1 that flakes of C glass are the only glass flakes which allow to produce high quality effect pigments.
WO 02/090448 A2 likewise discloses effect pigments on the basis of glass flakes with a thickness below 1 μm and which are coated with one or more layers with a high and/or low refractive index. Pursuant to the teaching of WO 02/090448 A2 quartz glass is the preferred glass as a substrate for effect pigments. Furthermore, any other glass composition having a softening point of ≧800° C. like e.g. Schott Duran or Supremax types can be used.
A glass type having a softening point of more than 800° C., disadvantageously, increases the costs for producing the glass flakes. As effect pigments are used in large amounts in printing inks, coating compositions, plastics, etc., it is desired to lower the costs for production of glass flakes in order to be able to economically produce effect pigments.
EP 1 340 715 discloses glass flakes onto which titanium oxide of rutile type is deposited. To promote the fixing of the titanium oxide of rutile type a compound selected from the group of platinum, palladium and palladium compound can be applied to the glass flake first. According to the teaching there are no particular limitations on the type of glass used for the glass flake substrate.
DE 10 2004 039 554 A1 discloses pearlescent pigments on the basis of glass flakes. DE 10 2004 039 554 A1, like EP 1 340 715, teaches that the composition of the glass flake substrate is of no importance for the properties of use.
JP 2005-187782 A teaches to use pearlescent pigments on the basis of a glass flake substrate wherein the glass flake substrate has a A-glass or E-glass composition which do not contain boric acid or lead oxide at all.
Moreover, pearlescent pigments based on Al2O3-platelets are known. However, these pearlescent pigments are available only in sizes below about 30 μm. Manufactures of pearlescent pigments for the cosmetic market, however, want to offer a product program of various sizes based on a single source of substrate. SiO2-flakes are expensive to produce and quite difficult to be produced in constant quality. Furthermore they exhibit extremely high colourflops which are not always demanded. Glass flakes would be the best choice as artificial substrates for effect pigments, especially for pearlescent pigments, because they can be produced reproducibly at fairly low costs and are achievable in various sizes and thickness.
Although glass flakes have been shown to be able to replace mica or Al2O3 platelets as substrate for pearlescent pigments, there is still a need for further optimised flakes yielding effect pigments with better gloss and skin feeling properties, especially when used in cosmetics.